drm/amd/display: Check dc_stream_state before it is used

dc_state_get_stream_status dc_state_get_paired_subvp_stream and other
functions can return null, and therefore null must be checked before
status can be used.

This fixes 21 NULL_RETURNS issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>